The present invention relates to a studless tire having a plurality of block rows in the tread portion, and more particularly to a studless tire having both excellent on-the-ice performances and excellent on-the-snow performances.
Various studless tires suitable for traveling on icy and snow-covered roads have been proposed, for example, in JP-A-2003-63211. A plurality of blocks provided with sipes are formed in the tread portion of studless tires. The coefficient of friction between the blocks and an icy road is enhanced by the edges of sipes. The sipes also serve to absorb water on an icy road. By a synergistic action of them, studless tires can exhibit both high driving force and high braking force on an icy road.
However, it is not easy to achieve both excellent on-the-ice performances (performances on an icy road) and excellent on-the-snow performances (performances on a snow-covered road) even by studless tires. A reason is that it is effective for running on an icy road to increase the ground contact area to thereby increase the frictional force, but it is required for running on a snow-covered road to increase the groove area to thereby enhance the snow-expelling performance. These requirements are contradictory to each other.
Accordingly, it is an object of the present invention to provide a winter tire called studless tire which have both excellent on-the-ice performances and excellent on-the-snow performances.
This and other objects of the present invention will become apparent from the description hereinafter.